It's not well documented, but the /tmp/builtin directory is intended for such purpose. So you'd have /tmp/builtin/optional and such, with no tce= bootcode.
A tce dir implies you can change things and have the changes stick, which is why it's only supposed to be on persistent, writable media such as hd, or when netbooting, nfs. Using tce= for a non-persistent dir may cause some slowdowns due to unnecessary operations on boot and shutdown.